For many years the electronics industry has used belt type furnaces for high volume heating applications. For an application, such as chip join, the operation is characterized by loading many parts on the belt, followed by continuous movement of the belt through the furnace's heating areas. It is also very important that the furnace provides a uniform temperature across the belt so that each individual part reaches the same temperature during processing. During a typical high volume heating applications intra-part gradients and short dwell times never becomes a problem, because the parts are small and are easily heated. For these applications the total heating system has evolved to a point where it is very reliable and reasonably priced.
U.S. Pat. No. 4,554,437 (Wagner et al.) discloses a continuous speed belt type tunnel oven which allows a user to select different top and bottom temperatures within each of the plural cooking zones.
U.S. Pat. No. 4,886,954 (Yu et al.) discloses a hot wall diffusion furnace and a method for operating the furnace. Yu et al. disclose that the heating elements in the upper section of the furnace be connected to one circuit, and the heating elements of the lower section of the furnace be connected to a second circuit, and that each circuit be controlled in response to the temperature in that section, so that uniform temperature can be obtained in the processing chamber.
U. S. Pat. No. 4,950,870 (Mitsuhashi et al.) discloses a heat-treating apparatus having at least three heaters and the power to these heaters can be supplied from independent power sources so that the heating temperatures of the individual heaters can be freely adjusted. Additionally, the multiple heaters in the vertical furnace attain a uniform heat distribution throughout the heating zone.
U.S. Pat. No. 4,966,547 (Okuyama et al.) discloses a heat treatment method using a zoned tunnel furnace. The furnace has roller conveyer and each of the zones in the furnace walls are provided with electric resistance heating wires. The heaters in each zone are under programmed control, independent of the heaters in the other zones. Similarly, the roller conveyer in each zone can be driven independent of the roller conveyer in the other zones by programmable controllers.
U. S. Pat. No. 4,982,347 (Rackerby et al.) discloses a process and apparatus for producing temperature profiles in a workpiece as it passes through a belt furnace. Each of the heaters has their own separate thermostats, which enables the temperature of each heater to be separately set. Thus a workpiece can be subjected to a temperature profile which varies from heater to heater along the passageway.
The parts or products using conventional belt type furnaces have changed over time. Some of the parts have been getting larger, and it has become increasingly difficult to do the same type of processing on the larger parts, as done by the furnaces known in the art. Because of the thermal mass or thermal weight some of the larger parts resist being heated quickly. Another factor is that newer and different materials are being used to make these parts and these newer materials require a different heating regimes. These issues are further compounded by the fact that now closer temperature control and lower intra-part gradients are being required by the electronics industry, and this has made the conventional belt furnace only marginally acceptable.
The manufacturers of conventional belt type furnaces have made quite a few upgrades to their furnaces in response to the industrial needs. Some upgrades include providing better and more efficient gas flows. Others have provided improved zone separation. And, still others are providing better cooling in the cool down section. Most of these changes are required because the parts or products are less tolerant to thermal process irregularities and the resultant mechanical stress.
For the larger parts it was observed that when these large parts were run in conventional belt furnaces they heat around the periphery faster than in the center. It was noticed that this temperature gradient was as large as 50.degree. C. or larger. Some of the conventional belt type furnace manufacturers have responded to this problem by providing left, right, as well as center, trim or adjustment control in their furnaces to try to solve this problem. This provides heat to the left, right and center of the part, but provides no heat adjustment to the leading or trailing edge of the part. And, nothing has been done to control front to back and intra-part gradients of the part or substrate.
For the above-mentioned reasons, processes such as chip join and pin braze on larger products cannot always be processed within specification using the belt type conventional furnaces. And, those parts that are processed, are processed at the full tolerance of the specification.
For example, the chip join process is characterized by two main parameters. The chip join process, is a process where an I. C. chip is joined to a substrate or carrier, typically using a plurality of solder balls. First, the part, such as a chip and the substrate, must go from the melting point of the alloy (Mp), i.e., the Mp of the solder balls, to a greater temperature (e.g. over 30.degree. C.) and then back to Mp. Secondly, this raising and lowering of the temperature for the chip join process must be done in minutes. This has not been a problem for most belt furnaces, as long as the part or product or carrier is in the 50 mm by 50 mm size range. Products or substrates in the 100 mm by 100 mm size range begin to present a problem, due to their large thermal mass, making it very difficult to heat to the desired temperature and then cool it to its original temperature in minutes. Furthermore, rapid heating of these parts or carriers introduces large temperature gradients. These gradients as discussed elsewhere are as large as 50.degree. C. or larger.
Another problem faced in the use of conventional belt furnaces is that when flux or similar contaminants are used in a conventional belt furnace they get deposited on the walls of the furnace creating a contamination problem. The reason for this problem is the fact that in the conventional belt furnaces the process gases enter at the end of the last heated zone and they flow toward the front or load end of the furnace where it normally exits. Flux and similar contaminants are used in many processes, such as a soldering process. Similarly, there are other solvents which evaporate from the surface of the part, as the part is heated, and they enter the flow of the gases in the furnace, flowing from the hotter end or area to a colder area. This causes the vaporized solvents and similar other material to condense on cooler furnace areas and this collects as contamination. One of the main reason to have the gas flow in this direction in a conventional belt furnace is to use the hot gas to heat small parts in the front or load end of the furnace by convection. On small parts this scheme provides more uniform heating. This contamination of the furnace wall problem does not exists in the inventive furnace.
The belt furnace of this invention overcomes the above-mentioned and other shortcomings of the conventional belt type furnaces.